Conventional hot-rolled high-strength steel sheets for working have a carbon content of about 0.03% or more and are usually manufactured by utilizing the strengthening of the structure through quenching by making use of the carbon and further precipitation hardening through addition of solid-solution strengthening elements, such as manganese, silicon or phosphorus, and the use of carbonitrides of titanium, niobium, etc.
The workability, particularly ductility of the high strength steel sheet thus manufactured decreases with an increase in the tensile strength. Therefore, it is impossible to ensure high strength while maintaining high workability.
There exists n technique which can sufficiently meet the above-described conflicting requirements of ensuring high strength while maintaining high workability. One of the techniques considered ideal for solving the above-described problem is that the steel sheet has low strength and high workability, particularly sufficiently high ductility, during cold work deformation, while the strength of the work produced by working can be increased after the completion of the working. If this technique can be realized, it is possible to produce a final product in the form of a complicated worked part which is also strong. Examples of the technique according to this ideal include a process described in Japanese Patent Publication No. 17049/1982. This process utilizes a change in the state of copper from that of solid solution to that of precipitation. That is, in this process, the steel sheet is worked while it is in a low strength state and thereafter the worked part is heat-treated to precipitate copper, thereby increasing the strength of the worked part.
However, the technique described in Japanese Patent Publication No. 17049/1982 with respect to an increase in the strength of the steel sheet through heat treatment of copper in the form of solid solution for causing precipitation, and conditions for the heat treatment, have been well known in the art for a long time. For example, they are expressly described in "Alloys of Iron and Copper" published by McGraw-Hill Book Company, Inc., 1934.
There is an ever-increasing demand from users with respect to an increase in the characteristics of the material for a recent hot-rolled steel sheet having high workability. This is because there are an increasing demand for parts having a complicated shape requiring high work deformation and an ever-increasing need on the side of steel sheet users with respect to cost reduction through a reduction in the number of the steps of working for deformation as much as possible. Therefore, the above-described process described in Japanese Patent Publication No. 17049/1982 does not meet at all the above-described demands of the steel sheet users.
One of the recent strong demands of the steel sheet users is to increase the strength of the final product to a great extent. For example, in recent years, there is a demand for the production of a part from a steel sheet having a tensile strength as high as at least 60 kgf/mm.sup.2, which part had a tensile strength of 45 kgf/mm.sup.2 when produced in the prior art. This renders necessary to develop a process which enables the manufacture of a steel sheet having not only very high strength but also high workability.
Further, there is a demand for a steel sheet which exhibits very high deformation working performance during deformation working. This is attributed to the fact that since final parts having more and more complicated shapes are desired, a steel sheet meeting this requirement should be provided. Moreover, there is also a strong demand on the users' side with respect to a reduction in the number of steps of working, which makes it necessary to provide a steel sheet having very high deformation working.
A further demand on the users' side is to simplify the step of heat treatment. It is a matter of course that the parts maker intending cost reduction has a need of further increasing the productivity through the completion of the heat treatment in a short period of time.
There is no prior art process meeting the these demands of the steel sheet users with respect to a new steel sheet. The present inventors have developed a process meeting the above-described demands.